This invention relates generally to a process for fabricating film structures, and more specifically to a process for depositing films on a carbon release layer and then oxidatively removing the release layer to leave a free-standing film.
There are a number of applications which require the formation of a thin or thick film structure in which the structure is totally free-standing or is anchored over a minor portion of its total extent to leave a cantilevered portion. In this context, both types of structures will be described as "free-standing". The totally free film structure, for example, is useful in the fabrication of thin polycrystalline sheets of silicon which are subsequently acted upon to improve the crystalline structure and then are used to fabricate semiconductor devices. Cantilevered structures are useful, for example, as beams, membranes, or diaphragms of accelerometers, strain gauges, and the like or as low capacitance crossovers for electrical interconnections in certain integrated circuits.
In the fabrication of solar cells, for example, large area, thin sheets or ribbons of silicon are provided with a PN junction for the photovoltaic generation of electricity in response to incident light. One way to provide the sheets of silicon is by the thermal expansion shear separation (TESS) process in which polycrystalline silicon is deposited onto a refractory substrate and then separated from the substrate by differences in the thermal expansion of the substrate and silicon. The TESS process is described, for example, in U.S. Pat. No. 4,370,288. The polycrystalline silicon sheets may then be improved in crystal quality, for example, by a ribbon-to-ribbon process such as that described in U.S. Pat. No. 4,410,471. The TESS process, however, leads to undesirable stresses in the silicon sheets, sheet breakage where imperfect separation between the sheet and substrate occurs, and low yield.
Cantilever beams of silicon or other refractory material are used in micro-mechanical structures such as accelerometers. In one application, for example, an air dielectric, parallel plate capacitor is formed between a substrate and a conductive cantilevered beam. Changes in acceleration are detected as a change in capacitance as the cantilever beam is caused to bend. The cantilever beam has been fabricated, for example, by depositing silicon or other refractory material on a coating overlying a substrate. After patterning the refractory material, the intermediate material is etched away to leave the end of the refractory material cantilevered with respect to the underlying substrate. Making cantilever beams in this manner, however, has been difficult because of the etch requirements of the intermediate material and limitations on the intermediate material because of temperatures encountered during the deposition of the refractory material.
Accordingly, a need existed for an improved method for providing free-standing films or sheets of refractory material which would overcome the difficulties attendant with the above-mentioned processes.
It is therefore an object of this invention to provide an improved process for fabricating freestanding film structures.
It is another object of this invention to provide a process for removing a carbon film from between two adjacent refractory materials.
It is another object of this invention to provide an improved process for fabricating sheets or shaped objects of silicon.